Glossary of Epidemiology Terms

Agent: A causative factor, such as a biological or chemical agent that must be present (or absent) in the environment for disease occurrence in a susceptible host.

Aggregate data: Data combined from several measurements or sets of measurements.

Analytic epidemiologic studies: Studies that examine groups of individuals in order to make comparisons and associations and to determine causal relationships; also known as cohort and case-control studies.

Bias: Any process or factor at any stage of a study from, its design to its execution to the application of information from the study, that produces results or conclusions that skew the results.

Case-control study: An analytic epidemiologic study design that assembles study groups after a disease has occurred; also called a retrospective study.

Cohort study: An analytic epidemiologic study type that assembles study groups before disease occurrence to observe and compare the rates of a health outcome over time; also called a prospective study.

Confounding factor: The distortion of the effect of one risk factor by the presence of another.

Correlation study: A descriptive epidemiologic study design used to compare aggregate populations for potential exposures of disease.

Cross- sectional survey: A descriptive epidemiologic study design that uses a representative sample of the population to collect information on current health status, personal characteristics, and potential risk factors or exposures at one point in time.

Descriptive epidemiologic studies: Epidemiologic study designs that contribute to the description of a disease or condition by examining the essential features of person, place, and time.

Environment: Internal and external factors that constitute the context for agent-host interactions; one's surroundings of any type.

Epidemic: A number of cases of an infectious agent or disease (outbreak) clearly in excess of the normally expected frequency of that disease in that population.

Exposure: The process by which an agent comes into contact with a person or animal in such a way that the person or animal may develop disease.

Incidence: The number of new cases of a disease diagnosed each year.

Intervention study: Epidemiologic study design that is experimental in nature and used to test a hypothesis about a cause-and-effect relationship.

Morbidity rate: A disease rate, specifically prevalence and incidence rates of diseases in a population in a specified time period.

Mortality rate: The number of deaths from all causes divided by the total population at a particular time and place.

Observational studies: Nonexperimental studies that describe, compare, and explain disease occurrence.

Prevalence: The total number of cases of the disease in the population at a given time divided by the number of individuals in the population.

Prevention:

Primary prevention: Inhibiting the development of disease before it occurs.

Secondary prevention: Also called "screening," refers to measures that detect disease before it is symptomatic.

Tertiary prevention: Focuses on people already affected by disease and attempt to reduce resultant disability and restore functionality.

Prospective study: An epidemiologic study design that assembles study groups before disease occurrence.

Random error: Is just that, random. It can occur during data collection (then called sampling error), coding, transfer, or analysis.

Relative risk: An epidemiologic measure of association that indicates the likelihood that an exposed group will develop a disease or condition relative to those not exposed.

Retrospective study: An epidemiologic study design that assembles study groups after disease occurrence.

Risk: The probability that an event, outcome, disease, or condition will develop in a specified time period.

Selection bias: The sampled group is biased in that it does not accurately represent the population it is intended to represent.

Sensitivity: The probability that an individual who has the disease of interest will have a positive screening test result. A test with high sensitivity has few false negatives, so you do not miss people with the disease.

Specificity: The probability that an individual who does not have the disease of interest will have a negative screening test result. A test with high specificity has few false positives so you do not identify people with no disease are rarely missed or told they don't have the disease.

• Diseased individuals with a positive screening test (true positives)
• Non diseased individuals with a positive screen (false positive)
• Diseased individuals with a negative screen ( false negative)
• Nondiseased individuals with a negative screen (true negative)

Systematic error: An error that is constant in a series of repetitions of the same experiment or observation. For example a scale could be set inaccurately.

Surveillance: The systematic collection and evaluation of all aspects of disease occurrence and spread, resulting in information that may be useful in the control of the disease.

Therapeutic trials: An epidemiologic intervention study design used to compare measures or interventions aimed at therapeutic benefits.

Validity: The validity of a study is dependent on the degree of systematic error. Validity is usually separated into two components:

Internal validity: Validity is dependent on the amount of error in measurements, including exposure, disease, and the associations between these variables.

External Validity: Means that the findings of the study can be accurately generalized to the population from which the sample was drawn (or even beyond that population to a more universal application).

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